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Proton auroras are widely observed on the dayside of Mars, identified as a significant intensity enhancement in the hydrogen Lyman alpha (121.6 nm) emission between 110 - 150 km altitudes. Solar wind protons penetrating as energetic neutral atoms into Mars thermosphere are thought to be primarily responsible for these auroras. Recent observations of spatially localized (patchy) proton auroras suggest a possible direct deposition of protons into Mars atmosphere during unstable solar wind conditions. Improving our understanding of proton auroras is therefore important for characterizing the solar wind interaction with Mars atmosphere. Here, we develop a first purely data-driven model of proton auroras using Mars Atmosphere and Volatile EvolutioN (MAVEN) in-situ observations and limb scans of Ly-alpha emissions between 2014 - 2022. We train an artificial neural network (ANN) that reproduces individual Lyman alpha intensities and relative Lyman alpha peak intensity enhancements with a Pearson correlation of 0.94 and 0.60 respectively for the test data, along with a faithful reconstruction of the shape of the observed Lyman alpha emission altitude profiles. By performing a SHapley Additive exPlanations (SHAP) analysis, we find that solar zenith angle, solar longitude, CO2 atmosphere variability, solar wind speed and temperature are the most important features for the modeled Lyman alpha peak intensity enhancements. Additionally, we find that the modeled peak intensity enhancements are high for early local time hours, particularly near polar latitudes, as well as weaker induced magnetic fields. Through SHAP analysis, we also identify the influence of biases in the training data and interdependecies between the measurements used for the modeling, and an improvement on those aspects can significantly improve the performance and applicability of the ANN model., Comment: 23 Pages, 17 Figures, 5 Tables, Accepted in PsJ

For the past several decades, numerous attempts have been made to model the climate of Mars with extensive studies focusing on the planet's dynamics and the understanding of its climate. While physical modeling and data assimilation approaches have made significant progress, uncertainties persist in comprehensively capturing and modeling the complexities of Martian climate. In this work, we propose a novel approach to Martian climate modeling by leveraging machine learning techniques that have shown remarkable success in Earth climate modeling. Our study presents a deep neural network designed to accurately model relative humidity in Gale Crater, as measured by NASA's Mars Science Laboratory ``Curiosity'' rover. By utilizing simulated meteorological variables produced by the Mars Planetary Climate Model, a robust Global Circulation Model, our model accurately predicts relative humidity with a mean error of 3\% and an $R^2$ score of 0.92. Furthermore, we present an approach to predict quantile ranges of relative humidity, catering to applications that require a range of values. To address the challenge of interpretability associated with machine learning models, we utilize an interpretable model architecture and conduct an in-depth analysis of its internal mechanisms and decision making processes. We find that our neural network can effectively model relative humidity at Gale crater using a few meteorological variables, with the monthly mean surface H$_2$O layer, planetary boundary layer height, convective wind speed, and solar zenith angle being the primary contributors to the model predictions. In addition to providing a fast and efficient method to modeling climate variables on Mars, this modeling approach can also be used to expand on current datasets by filling spatial and temporal gaps in observations., Comment: 13 pages, 7 figures

A potential crewed mission to Mars would require us to solve a number of problems, including how to protect astronauts against the devastating effects of energetic charged particles from Solar and Galactic sources. The radiation environment on Mars is of particular interest, since maintaining optimal absorbed doses by astronauts is crucial to their survival. Here, we give an overview of the conditions on Mars, as determined by theoretical models and in-situ measurements, and present the main proposed strategies to mitigate radiation exposure while on Mars. Specifically, we focus on the passive shielding technique. Several widely used materials, along with some innovative ones and combinations of those, are studied for their behavior against Solar Energetic Particle Events and Galactic Cosmic Rays in the Martian environment. For that purpose, we implement the GEANT4 package, a Monte-Carlo numerical model developed by CERN, which is specifically applied to simulate interactions of radiation with matter. A description of our model will be given, followed by outputs of the numerical model. We conclude that hydrogen-rich materials act as better attenuators, as expected, but other materials can be helpful against cosmic rays too., Comment: 5 pages, 7 figures, presented at the 73rd International Astronautical Congress (IAC)

Auroras are emissions in a planetary atmosphere caused by its interactions with the surrounding plasma environment. They have been observed in most planets and some moons of the solar system. Since their first discovery in 2005, Mars auroras have been studied extensively and is now a rapidly growing area of research. Since Mars lacks an intrinsic global magnetic field, its crustal field is distributed throughout the planet and its interactions with the surrounding plasma environment lead to a number of complex processes resulting in several types of auroras uncommon on Earth. Martian auroras have been classified as diffuse, discrete and proton aurora. With new capability of synoptic observations made possible with the Hope probe, two new types of auroras have been observed. One of them, which occurs on a much larger spatial scale, covering much of the disk, is known as discrete sinuous aurora. The other subcategory is one of proton auroras observed in patches. Further study of these phenomena will provide insights into the interactions between the atmosphere, magnetosphere and the surrounding plasma environment of Mars. We provide a brief review of the work done on the subject in the past 17 years since their discovery, and report new developments based on observations with Hope probe., Comment: 14 pages, 7 figures

The impact of exposure to astrophysical ionizing radiation on astronaut health is one of the main concerns in planning crewed missions to Mars. Astronauts will be exposed to energetic charged particles from Galactic and Solar origin for a prolonged period with little protection from a thin spacecraft shield in transit and from the rarefied Martian atmosphere when on the surface. Adverse impacts on astronaut health include, for example, Acute Radiation Syndrome, damage to the nervous system, and increased cancer risk. We rely on medical studies to assess the impact of enhanced radiation dose levels on various physiological systems and the overall health of astronauts. Using a combination of radiation measurements and numerical modeling with the GEANT4 package, we calculate the distribution of radiation dose in various human body organs for various expected scenarios simulated with a model human phantom. We suggest mitigation strategies, such as improved ways of shielding and dietary supplements, and make recommendations for the safety of astronauts in future crewed missions to Mars., Comment: Under Review

Ionizing radiation is known to have a destructive impact on biology by causing damage to the DNA, cells, and production of Reactive Oxygen Species (ROS) among other things. While direct exposure to high radiation dose is indeed not favorable for biological activity, ionizing radiation can, and in some cases is known to produce a number of biologically useful products. One such mechanism is the production of biologically useful products via charged particle-induced radiolysis. Energetic charged particles interact with surfaces of planetary objects such as Mars, Europa and Enceladus without much shielding from their rarefied atmospheres. Depending on the energy of said particles, they can penetrate several meters deep below the surface and initiate a number of chemical reactions along the way. Some of the byproducts are impossible to produce with lower-energy radiation (such as sunlight), opening up new avenues for life to utilize them. For each of these cases, we calculate the energy deposition rate as a function of depth, and estimate the energy availability for potential metabolic activity. We discuss various mechanisms through which life could support itself utilizing the byproducts of these ionizing radiation-induced reactions, such as chemoautotrophs using solvated electrons, extracellular electron transfer, and indirect electrophy to facilitate processes like carbon fixation, nitrogen fixation and sulfate reduction, and possibly for ATP production., Comment: 22 pages

The surface of Mars is bombarded by energetic charged particles of solar and cosmic origin with little shielding compared to Earth. As space agencies are planning for crewed missions to the red planet, a major concern is the impact of ionizing radiation on astronaut health. Keeping exposure below acceptable radiation dose levels is crucial for the health of the crew. In this study, our goal is to understand the radiation environment of Mars and describe the main strategies to be adopted to protect astronauts from the harmful impacts of cosmic radiation. Specifically, we investigate the shielding properties of various materials in the Martian radiation field using the Geant4 numerical model, after validating its accuracy with in-situ instrument measurements by MSL RAD. Our results indicate that composite materials such as types of plastic, rubber or synthetic fibers, have a similar response against cosmic rays and are the best shields. Martian regolith has an intermediate behavior and therefore could be used as an additional practical option. We show that the most widely used aluminum could be helpful when combined with other low atomic number materials., Comment: 26 pages, 13 figures, accepted by EPJP

For the first time, the Emirates Mars Infrared Spectrometer (EMIRS) instrument on board the Emirates Mars Mission (EMM) "Hope", is providing us with the temperature measurements of Mars at all local times covering most of the planet. As a result, it is now possible to compare surface temperature measurements made from orbit with those from the surface by rovers during the same time period. We use data of diurnal temperature variation from the Rover Environmental Monitoring Station (REMS) suite on board the Mars Science Laboratory (MSL) "Curiosity" rover, and the Mars Environmental Dynamics Analyzer (MEDA) suite on board the Mars 2020 "Perseverance" rover, between June and August 2021 and compare them with EMIRS observations and estimates of the Mars Climate Database (MCD) model. We show that although the overall trend of temperature variation is in excellent agreement across missions, EMIRS measurements are systematically lower at night compared to Mars 2020. The lower spatial resolution of EMIRS compared to the rovers and consequently lower average thermal inertia of the observed regions in this particular case primarily contributed to this discrepancy, among other factors. We discuss the implications of these results in improving our understanding of the Martian climate which would lead to better modeling of local weather prediction, useful for future robotic and crewed missions., Comment: Accepted for publication in MNRAS Letters

With a fleet of exploratory space missions on the horizon, the study of target specific biospheres is crucial for accurately determining the probability of the existence of microbial life on various planetary bodies and prioritising targets accordingly. Although previous studies have compared the potential habitability of objects in our solar system by bulk characteristics, it is less common that precise qualitative methods are developed for ranking candidates hospitable to microbial life on a local environment basis. In this review we create a planetary environmental database and use it to motivate a list of primary habitability candidates and essential criteria for microbial survival. We then propose a new method, the Microbial Habitability Index (MHI) which uses a metric of microbial survival factor values in target environments compared with appropriate Earth analogues to assess their potential for life. We arrive at a selection of eight primary candidates and from this set conclude that Europa, Mars, and Enceladus have the highest potential for facilitating microbial survival., Comment: 42 pages, 3 figures

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine if environments are habitable or not, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help to improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets., Comment: Published in Astrobiology, 21(8). arXiv admin note: substantial text overlap with arXiv:2007.05491

Solar Proton Events (SPEs) can cause abrupt and significant enhancements to the Martian surface radiation dose. Observations of the impact of SPEs on the Martian surface are available from satellites and surface detectors, but the data set is very limited in time, and the energy range is limited in scope, which makes it insufficient to estimate the impact of major events on the Martian surface. On the other hand, long-term data of SPEs impacting the Earth spanning a large energy range is widely available, and can be used to estimate the impact of major events on Mars on long timescales. Herein, we take major SPEs observed during the past several decades on Earth (1956 - 2014), along with PAMELA observations (2006 - 2014) and use the GEANT4 Monte Carlo code to calculate the Martian surface radiation dose. We study the contribution of proton fluence and spectral shape of events on the surface radiation dose and estimated the impact of possible major SPEs on the Martian surface in the future. These results have major implications for the planned human exploration of Mars. Overall we find that the radiation dose from extreme events can have a significant impact on astronaut health, and in rare, worst case scenarios, the estimated dose can even reach lethal levels., Comment: Submitted

Upcoming missions, including the James Webb Space Telescope, will soon characterize the atmospheres of terrestrial-type exoplanets in habitable zones around cool K- and M-type stars searching for atmospheric biosignatures. Recent observations suggest that the ionizing radiation and particle environment from active cool planet hosts may be detrimental for exoplanetary habitability. Since no direct information on the radiation field is available, empirical relations between signatures of stellar activity, including the sizes and magnetic fields of starspots, are often used. Here, we revisit the empirical relation between the starspot size and the effective stellar temperature and evaluate its impact on estimates of stellar flare energies, coronal mass ejections, and fluxes of the associated stellar energetic particle events., Comment: 14 pages, 4 figures, 2 tables, accepted for publication in The Astrophysical Journal (07/11/2020)

We analyze the additional effect on planetary atmospheres of recently detected gamma-ray burst afterglow photons in the range up to 1 TeV. For an Earth-like atmosphere we find that there is a small additional depletion in ozone versus that modeled for only prompt emission. We also find a small enhancement of muon flux at the planet surface. Overall, we conclude that the additional afterglow emission, even with TeV photons, does not result in a significantly larger impact over that found in past studies., Comment: Replaced with version to be published in MNRAS

Space weather plays an important role in the evolution of planetary atmospheres. Observations have shown that stellar flares emit energy in a wide energy range (10^30-10^38 ergs), a fraction of which lies in X-rays and extreme ultraviolet (XUV). These flares heat the upper atmosphere of a planet, leading to increased escape rates, and can result in atmospheric erosion over a period of time. Observations also suggest that primordial terrestrial planets can accrete voluminous H/He envelopes. Stellar radiation can erode these protoatmospheres over time, and the extent of this erosion has implications for the planet's habitability. We use the energy-limited equation to calculate hydrodynamic escape rates from these protoatmospheres irradiated by XUV stellar flares and luminosity. We use the Flare-Frequency Distribution of 492 FGKM stars observed with TESS to estimate atmospheric loss in Habitable Zone planets. We find that for most stars, luminosity-induced escape is the main loss mechanism, with a minor contribution from flares. However, flares dominate the loss mechanism of $\sim$20\% M4-M10 stars. M0-M4 stars are most likely to completely erode both their proto- and secondary atmospheres, and M4-M10 are least likely to erode secondary atmospheres. We discuss the implications of these results on planetary habitability., Comment: Accepted in MNRAS Letters

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency between them and different in function to those used by ecologists. In this white paper, we suggest a mass-energy habitability model as an example of how to adapt and expand the models used by ecologists to the astrobiology field. We propose to implement these models into a NASA Habitability Standard (NHS) to standardize the habitability objectives of planetary missions. These standards will help to compare and characterize potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science. The synergy between the methods used by ecologists and astrobiologists will help to integrate and expand our understanding of the habitability of Earth, the Solar System, and exoplanets., Comment: White Paper for the Planetary Science and Astrobiology Decadal Survey 2023-2032

There is growing evidence suggesting the presence of aqueous environment on ancient Mars, raising the question of the possibility of life in such an environment. Subsequently, with the erosion of the Martian atmosphere resulting in drastic changes in its climate, surface water disappeared, shrinking habitable spaces on the planet, with only a limited amount of water remaining near the surface in form of brines and water-ice deposits. Life, if it ever existed, would have had to adapt to harsh modern conditions, which includes low temperatures and surface pressure, and high radiation dose. Presently, there is no evidence of any biological activity on the planet's surface, however, the subsurface environment, which is yet to be explored, is less harsh, has traces of water in form of water-ice and brines, and undergoes radiation-driven redox chemistry. I hypothesize that Galactic Cosmic Ray (GCR)-induced radiation-driven chemical disequilibrium can be used for metabolic energy by extant life, and host organisms using mechanisms seen in similar chemical and radiation environments on Earth. I propose a GCR-induced radiolytic zone, and discuss the prospects of finding such life with Rosalind Franklin rover of the ExoMars mission., Comment: 13 pages, 2 figures

The Atmospheric Radiation Interaction Simulator (AtRIS) was used to model the altitude-dependent Venusian absorbed dose and the Venusian dose equivalent. For the first time, we modeled the dose rates for different shape-, size-, and composition-mimicking detectors (phantoms): a CO$_2$-based phantom, a water-based microbial cell, and a phantom mimicking human tissue. Based on a new model approach, we give a reliable estimate of the altitude-dependent Venusian radiation dose in water-based microorganisms here for the first time. These microorganisms are representative of known terrestrial life. We also present a detailed analysis of the influence of the strongest ground-level enhancements measured at the Earth's surface, and of the impact of two historic extreme solar events on the Venusian radiation dose. Our study shows that because a phantom based on Venusian air was used, and because furthermore, the quality factors of different radiation types were not taken into account, previous model efforts have underestimated the radiation hazard for any putative Venusian cloud-based life by up to a factor of five. However, because we furthermore show that even the strongest events would not have had a hazardous effect on putative microorganisms within the potentially habitable zone (51 km - 62 km), these differences may play only a minor role.

The discovery of terrestrial exoplanets orbiting in habitable zones around nearby stars has been one of the significant developments in modern astronomy. More than a dozen such planets, like Proxima Centauri b and TRAPPIST-1 e, are in close-in configurations and their proximity to the host star makes them highly sensitive to stellar activity. Episodic events such as flares have the potential to cause severe damage to close-in planets, adversely impacting their habitability. Flares on fast rotating young M stars occur up to 100 times more frequently than on G-type stars which makes their planets even more susceptible to stellar activity. Stellar Energetic Particles (SEPs) emanating from Stellar Proton Events (SPEs) cause atmospheric damage (erosion and photochemical changes), and produce secondary particles, which in turn results in enhanced radiation dosage on planetary surfaces. We explore the role of SPEs and planetary factors in determining planetary surface radiation doses. These factors include SPE fluence and spectra, and planetary column density and magnetic field strength. Taking particle spectra from 70 major solar events (observed between 1956 and 2012) as proxy, we use the GEANT4 Monte Carlo model to simulate SPE interactions with exoplanetary atmospheres, and we compute surface radiation dose. We demonstrate that in addition to fluence, SPE spectrum is also a crucial factor in determining the surface radiation dose. We discuss the implications of these findings in constraining the habitability of terrestrial exoplanets., Comment: Accepted in MNRAS Letters

Duderstadt et al. [2016b] comment that the Melott et al. [2016] study of nitrate formation by solar proton events (SPEs) and comparison with the ice core archive is "fundamentally flawed," because it does not include pre-existing HNO3 in the stratosphere. We show that they exaggerate both the enhancement predicted by our findings and pre-industrial HNO3 levels in their model, and fail to prove this assertion. Our feasibility study matched expected SPE nitrate production with ground truth measurements. It is not clear that their approach is more realistic and absence of a detailed mechanism does not disprove our results. Models can be no better than the information they are provided and in this case there continue to be significant unknowns and uncertainties, especially in the role of polar stratospheric clouds (PSCs) and possible interactions with cosmic rays that constitute lower boundary conditions. Duderstadt et al. [2014; 2016a] used incomplete, poorly-constrained and incorrect initial and boundary conditions, and they continue to advocate on the basis of uncertain results. Meanwhile, Smart et al. [2014] identified a series of ice core nitrate spikes that have since been confirmed in 10Be by McCracken and Beer [2015]. Melott et al. [2016] computationally reproduced the ionization profile of the only major balloon measurements to date. We show that our calculated nitrate enhancement is consistent with measured results, given current levels of uncertainty, and that extreme SPEs can potentially produce occasional nitrate spikes with hundreds of percent increases. Instead of repeating old arguments to dismiss nitrates as proxies of SPEs, it is past time for a dedicated, fine-resolution, multi-parameter, replicate ice core field campaign to resolve this debate., Comment: Invited reply, submitted to JGR-Atmospheres

Kepler observations have uncovered the existence of a large number of close-in exoplanets and serendipitously of stellar superflares with emissions several orders of magnitude higher than those observed on the Sun. The interaction between the two and its implications on planetary habitability is of great interest to the community. Stellar Proton Events interact with planetary atmospheres, generate secondary particles and increase the radiation dose on the surface. This effect is amplified for close-in exoplanets and can be a serious threat to potential planetary life. Monte Carlo simulations are used to model the SPE-induced particle radiation dose on the surface of such exoplanets. The results show a wide range of surface radiation doses on planets in close-in configurations with varying atmospheric column depths, magnetic moments and orbital radii. It can be concluded that for close-in exoplanets with sizable atmospheres and magnetospheres, the radiation dose contributed by stellar superflares may not be high enough to sterilize a planet (for life as we know it) but can result in frequent extinction level events. In light of recent reports, the interaction of hard-spectrum SPEs with the atmosphere of Proxima Centauri b is modeled and implications on its habitability is discussed., Comment: MNRAS Letters, in press

Mars is believed to have had a substantial atmosphere in the past. Atmospheric loss led to depressurization and cooling, and is thought to be the primary driving force responsible for the loss of liquid water from its surface. Recently, MAVEN observations have provided new insight into the physics of atmospheric loss induced by ICMEs and solar wind interacting with the Martian atmosphere. In addition to solar radiation, it is likely that its atmosphere has been exposed to radiation bursts from high-energy astrophysical sources which become highly probable on timescales of ~Gy and beyond. These sources are capable of significantly enhancing the rates of photoionization and charged particle-induced ionization in the upper atmosphere. We use Monte Carlo simulations to model the interaction of charged particles and photons from astrophysical sources in the upper Martian atmosphere and discuss its implications on atmospheric loss. Our calculations suggest that the passage of the solar system though dense interstellar clouds is the most significant contributor to atmospheric loss among the sources considered here., Comment: Published in MNRAS Letters (accepted version)

Solar energetic particles ionize the atmosphere, leading to production of nitrogen oxides. It has been suggested that some such events are visible as layers of nitrate in ice cores, yielding archives of energetic, high fluence solar proton events (SPEs). There has been controversy, due to slowness of transport for these species down from the upper stratosphere; past numerical simulations based on an analytic calculation have shown very little ionization below the mid stratosphere. These simulations suffer from deficiencies: they consider only soft SPEs and narrow energy ranges; spectral fits are poorly chosen; with few exceptions secondary particles in air showers are ignored. Using improved simulations that follow development of the proton-induced air shower, we find consistency with recent experiments showing substantial excess ionization down to 5 km. We compute nitrate available from the 23 February 1956 SPE, which had a high fluence, hard spectrum, and well-resolved associated nitrate peak in a Greenland ice core. For the first time, we find this event can account for ice core data with timely (~ 2 months) transport downward between 46 km and the surface, thus indicating an archive of high fluence, hard spectrum SPE covering the last several millennia. We discuss interpretations of this result, as well as the lack of a clearly-defined nitrate spike associated with the soft-spectrum 3-4 August 1972 SPE. We suggest that hard-spectrum SPEs, especially in the 6 months of polar winter, are detectable in ice cores, and that more work needs to be done to investigate this., Comment: JGR Atmospheres, in press

Photosynthesis is a mechanism developed by terrestrial life to utilize the energy from photons of solar origin for biological use. Subsurface regions are isolated from the photosphere, and consequently are incapable of utilizing this energy. This opens up the opportunity for life to evolve alternate mechanisms for harvesting available energy. Bacterium Candidatus Desulforudis audaxviator, found 2.8 km deep in a South African mine, harvests energy from radiolysis, induced by particles emitted from radioactive U, Th and K present in surrounding rock. Another radiation source in the subsurface environments is secondary particles generated by Galactic Cosmic Rays (GCRs). Using Monte Carlo simulations, it is shown that it is a steady source of energy comparable to that produced by radioactive substances, and the possibility of a slow metabolizing life flourishing on it cannot be ruled out. Two mechanisms are proposed through which GCR-induced secondary particles can be utilized for biological use in subsurface environments: (1) GCRs injecting energy in the environment through particle-induced radiolysis, and (2) organic synthesis from GCR secondaries interacting with the medium. Laboratory experiments to test these hypotheses are also proposed. Implications of these mechanisms on finding life in the Solar System and elsewhere in the Universe are discussed., Comment: Accepted for publication in the Journal of the Royal Society Interface

Muons serve as the best probes of the physics of hadronic interactions in the upper atmosphere because of their simple physics. All the properties of detected muons, such as their energy and angle of incidence, are governed by the properties of their parent hadrons. The angular distribution of the detected muons is a result of a superposition of multiple effects, and is governed by a number of parameters, such as the cutoff rigidity (magnetic field), atmospheric attenuation, interaction cross sections, production height, multiplicity and pseudorapidity. Since particle interactions are handled by the low and high-energy hadronic interaction models in air shower simulations, subtle differences in their physics should manifest in the differences in the angular distribution. Here, we compare air shower simulations with experimental data and investigate how the choice of hadronic interaction models in air shower simulations affects the angular distribution of cosmic ray muons.

Gamma rays are the best probes to study high-energy particle interactions occurring in astrophysical environments. Space based instruments such as Fermi Large Area Telescope (Fermi LAT) and ground based experiments such as VERITAS, H.E.S.S. and MAGIC have provided us with valuable data on various production mechanisms of gamma rays within our Galaxy and beyond. Depending on astronomical conditions, gamma rays can be produced either by hadronic or leptonic interactions. In this paper, we probe the production of gamma rays by the hadronic channel where gamma rays are primarily produced by the decay of secondary neutral pions and $\eta$ mesons from proton-proton interactions in a wide energy range. We use state of the art high-energy hadronic interaction models, calibrated with the new LHC results and widely used in ground based ultra-high energy air shower experiments. We also compare SIBYLL 2.1, QGSJET-II-04 and EPOS LHC models and provide lookup tables which can be used by researchers to model gamma ray production from the hadronic channel and ultimately extract the underlying proton spectrum from gamma ray observations.

This past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground and space-based instruments, and exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets, falling in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker planetary magnetic fields, especially in case of super earths. Such exoplanets are subjected to a high flux of Galactic Cosmic Rays (GCRs) due to their weak magnetic moments. GCRs are energetic particles of astrophysical origin, which strike the planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the planetary surface and contribute to the radiation dose. Along with the magnetic field, another factor governing the radiation dose is the depth of the planetary atmosphere. The higher the depth of the planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the sub-surface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We explore the dependence of the GCR induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, finding that the latter is the decisive factor for the protection of a planetary biosphere., Comment: Accepted for publication in Astrobiology

Gamma ray bursts (GBRs) are a class of highly energetic explosions emitting radiation in a very short timescale of a few seconds and with a very narrow opening angle. Although, all GRBs observed so far are extragalactic in origin, there is a high probability of a GRB of galactic origin beaming towards the Earth in the past ~ 0.5 Gyr. Such an intense burst of gamma rays would ionize the atmosphere and deplete the ozone layer. With depleted ozone, there will be an increased flux of solar UVB on the Earth's surface with harmful biological effects. In addition to the atmospheric damage, secondary particles produced by gamma ray-induced showers will reach the surface. Amongst all secondary particles, muons dominate the ground-level secondary particle flux (99% of the total number of particles) and are potentially of biological significance. Using the Monte Carlo simulation code CORSIKA, we modeled the air showers produced by gamma ray primaries up to 100 GeV. We found that the number of muons produced by hypothetical galactic GRBs significantly increases the total muon flux. However, since the muon production efficiency is extremely low for photon energy below 100 GeV, and because GRBs radiate strongly for only a very short time, we find that the biological radiation dose from secondary muons is negligible. The main mechanism of biological damage from GRBs is through solar UVB irradiation from the loss of ozone in the upper atmosphere., Comment: Published in the International Journal of Astrobiology

"The investigation into the possible effects of cosmic rays on living organisms will also offer great interest." - Victor F. Hess, Nobel Lecture, December 12, 1936 High-energy radiation bursts are commonplace in our Universe. From nearby solar flares to distant gamma ray bursts, a variety of physical processes accelerate charged particles to a wide range of energies, which subsequently reach the Earth. Such particles contribute to a number of physical processes occurring in the Earth system. A large fraction of the energy of charged particles gets deposited in the atmosphere, ionizing the atmosphere, causing changes in its chemistry and affecting the global electric circuit. Remaining secondary particles contribute to the background dose of cosmic rays on the surface and parts of the subsurface region. Life has evolved over the past ~ 3 billion years in presence of this background radiation, which itself has varied considerably during the period. As demonstrated by the Miller-Urey experiment, lightning plays a very important role in the formation of complex organic molecules, which are the building blocks of more complex structures forming life. There is growing evidence of increase in the lightning rate with increasing flux of charged particles. Is there a connection between enhanced rate of cosmic rays and the origin of life? Cosmic ray secondaries are also known to damage DNA and cause mutations, leading to cancer and other diseases. It is now possible to compute radiation doses from secondary particles, in particular muons and neutrons. Have the variations in cosmic ray flux affected the evolution of life on earth? We describe the mechanisms of cosmic rays affecting terrestrial life and review the potential implications of the variation of high-energy astrophysical radiation on the history of life on earth., Comment: Published in Astroparticle Physics

Under current conditions, the cosmic ray spectrum incident on the Earth is dominated by particles with energies < 1 GeV. Astrophysical sources including high energy solar flares, supernovae and gamma ray bursts produce high energy cosmic rays (HECRs) with drastically higher energies. The Earth is likely episodically exposed to a greatly increased HECR flux from such events, some of which lasting thousands to millions of years. The air showers produced by HECRs ionize the atmosphere and produce harmful secondary particles such as muons and neutrons. Neutrons currently contribute a significant radiation dose at commercial passenger airplane altitude. With higher cosmic ray energies, these effects will be propagated to ground level. This work shows the results of Monte Carlo simulations quantifying the neutron flux due to high energy cosmic rays at various primary energies and altitudes. We provide here lookup tables that can be used to determine neutron fluxes from primaries with total energies 1 GeV - 1 PeV. By convolution, one can compute the neutron flux for any arbitrary CR spectrum. Our results demonstrate that deducing the nature of primaries from ground level neutron enhancements would be very difficult., Comment: 8 pages, 3 figures, JGR - in press

A ~ 62 My periodicity in fossil biodiversity has been observed in independent studies of paleontology databases over ~0.5Gy. The period and phase of this biodiversity cycle coincides with the oscillation of our solar system normal to the galactic disk with an amplitude ~70 parsecs and a period ~64 My. Our Galaxy is falling toward the Virgo cluster, forming a galactic shock at the north end of our galaxy due to this motion, capable of accelerating particles and exposing our galaxy's northern side to a higher flux of cosmic rays. These high-energy particles strike the Earth's atmosphere initiating extensive air showers, ionizing the atmosphere by producing charged secondary particles. Secondary particles such as muons produced as a result of nuclear interactions are able to reach the ground and enhance the biological radiation dose. Using a Monte Carlo simulation package CORSIKA, we compute the biological dose resulting from enhanced muon exposure from cosmic rays and discuss their implications for terrestrial biodiversity variations., Comment: Accepted for publication in Geophysical Research Letters

Messaging to extraterrestrial intelligence (METI) is a branch of study concerned with constructing and broadcasting a message toward habitable planets. Since the Arecibo message of 1974, the handful of METI broadcasts have increased in content and complexity, but the lack of an established protocol has produced unorganized or cryptic messages that could be difficult to interpret. Here we outline the development of a self-consistent protocol for messaging to extraterrestrial intelligence that provides constraints and guidelines for the construction of a message in order to maximize the probability that the message effectively communicates. A METI protocol considers several factors including signal encoding, message length, information content, anthropocentrism, transmission method, and transmission periodicity. Once developed, the protocol will be released for testing on different human groups worldwide and across cultural boundaries. An effective message to extraterrestrials should at least be understandable by humans, and releasing the protocol for testing will allow us to improve the protocol and develop potential messages. Through an interactive website, users across the world will be able to create and exchange messages that follow the protocol in order to discover the types of messages better suited for cross-cultural communication. The development of a METI protocol will serve to improve the quality of messages to extraterrestrials, foster international collaboration, and extend astrobiology outreach to the public., Comment: Accepted for publication in Space Policy

On geological timescales, the Earth is likely to be exposed to an increased flux of high energy cosmic rays (HECRs) from astrophysical sources such as nearby supernovae, gamma ray bursts or by galactic shocks. Typical cosmic ray energies may be much higher than the ~ 1 GeV flux which normally dominates. These high-energy particles strike the Earth's atmosphere initiating an extensive air shower. As the air shower propagates deeper, it ionizes the atmosphere by producing charged secondary particles. Secondary particles such as muons and thermal neutrons produced as a result of nuclear interactions are able to reach the ground, enhancing the radiation dose. Muons contribute 85% to the radiation dose from cosmic rays. This enhanced dose could be potentially harmful to the biosphere. This mechanism has been discussed extensively in literature but has never been quantified. Here, we have developed a lookup table that can be used to quantify this effect by modeling terrestrial muon flux from any arbitrary cosmic ray spectra with 10 GeV - 1 PeV primaries. This will enable us to compute the radiation dose on terrestrial planetary surfaces from a number of astrophysical sources.

Variation in high energy cosmic rays (HECRs) has been proposed to explain a 62 My periodicity in terrestrial fossil biodiversity. It has been suggested that the infall of our galaxy toward the Virgo cluster could generate an extragalactic shock, accelerating charged particles and exposing the earth to a flux of high energy cosmic rays (HECRs). The oscillation of the Sun perpendicular to the galactic plane could induce 62 My periodicity in the HECR flux on the Earth, with a magnitude much higher than the Galactic cosmic ray change we see in a solar cycle. This mechanism could potentially explain the observed 62 My periodicity in terrestrial biodiversity over the past 500 My. In addition to direct effects on life from secondaries, HECRs induced air showers ionize the atmosphere leading to changes in atmospheric chemistry and microphysical processes that can lead to cloud formation including low altitude cloud cover. An increase in ionization changes the global electric circuit which could enhance the formation of cloud condensation nuclei (CCN) through microphysical processes such as electroscavenging and ion mediated nucleation, leading to an increase in the cloud cover. This could increase the albedo and reduce the solar flux reaching the ground, reducing the global temperature. Using an existing model, we have calculated the enhancement in atmospheric ionization at low altitudes resulting from exposure to HECRs. We use a conservative model to estimate the change in low altitude cloud cover from this increased ionization., Comment: 10 pages, 1 figure

A variety of events such as gamma-ray bursts and supernovae may expose the Earth to an increased flux of high-energy cosmic rays, with potentially important effects on the biosphere. Existing atmospheric chemistry software does not have the capability of incorporating the effects of substantial cosmic ray flux above 10 GeV . An atmospheric code, the NASA-Goddard Space Flight Center two-dimensional (latitude, altitude) time-dependent atmospheric model (NGSFC), is used to study atmospheric chemistry changes. Using CORSIKA, we have created tables that can be used to compute high energy cosmic ray (10 GeV - 1 PeV) induced atmospheric ionization and also, with the use of the NGSFC code, can be used to simulate the resulting atmospheric chemistry changes. We discuss the tables, their uses, weaknesses, and strengths., Comment: In press: Journal of Cosmology and Astroparticle Physics. 6 figures, 3 tables, two associated data files. Major revisions, including results of a greatly expanded computation, clarification and updated references. In the future we will expand the table to at least EeV levels.